Beta-alumina polycrystalline ceramics

ABSTRACT

A method of preparing a Beta-alumina polycrystalline ceramic comprises firing a mix consisting essentially of by weight: Li2O - 0.7 - 1.5% Na2O - 8.3 - 8.9% MgO - 0.5 - 2.0% Al2O3 - balance.

United States Patent [191 Duncan et al.

[451 Oct. 16, 1973 BETA-ALUMINA POLYCRYSTALLINE CERAMICS Inventors: James H. Duncan; Brian K. Hick,

both of London, England Assignee: British Railway Board, London,

England Filed: Jan. 27, 1972 Appl. No.: 221,386

Foreign Application Priority Data References Cited UNITED STATES PATENTS 10/1970 Dziecivch et al. 136/153 X 10/ 1969 Tennenhouse 12/1971 Charles... 106/65 Primary Examiner-Helen M. McCarthy Attorney-Elliott I. Pollock et al.

ABSTRACT A method of preparing a Beta-alumina polycrystalline ceramic comprises firing a mix consisting essentially of by weight:

Li O 0.7 1.5% Na O 8.3 8.9% MgO 0.5 2.0% A1 0 balance.

10 Claims, No Drawings BETA-ALUMINA POLYCRYSTALLINE CERAMICS This invention relates to Beta-alumina polycrystalline ceramics. Such polycrystalline ceramics can, depending upon their composition and method of preparation, either take the form whose basic empirical formula is NaAl Ou or NaAI O or a form which is an aggregate of the two.

Such polycrystalline ceramics are suitable for use as an electrolyte in apparatus of the kind involving the transport of sodium ions by diffusion through a solid electrolyte, for example in electric cells or batteries of the sodium/sulphur type.

From the work of Thery and Briacon and of Yamaguchi it is known that the layer structure of crystalline Beta-alumina NaAl50g is similar to di 6f Betaalumina NaAl O but with a lower degree of crystal symmetry. The layer structure of the ceramic is composed of alternate spinel layers or blocks and sodium layers. The spinel layers consist of aluminum and oxygen atoms arranged as in gamma alumina and the sodium layer contains sodium and oxygen atoms. The unit cell of the structure repeats every second layer in NaAl O and every third layer in NaAl5OR.

It has already been proposed to format Beta-alumina polycrystalline ceramic using the basic constituents A12 03, N320, LiZQ MgQJn the original mix precursors of U and MgO may be used, for example LiNO: and

MgCO which convert to Li O and MgO during the for-' mation of the ceramic. The present invention is concerned with Beta-alumina polycrystalline ceramics formed from these four constituents, the object of the invention being to produce a Beta-alumina which is more stable than hitherto in atmospheric conditions without detriment to the other properties required for its use as an electrolyte.

In research leading to this invention it has been discovered that providing that the composition is within defined limits a Beta-alumina consisting of between 90 and 100 percent NaAl O type is produced and that such a Beta-alumina as compared with known Betaalumina even of the basic NaAl O form is stable in atmospheric conditions and has. the high conductivity of sodium ions associated with NaAl O as compared with NaAI O According to this invention a method of preparing a Beta-alumina polycrystalline ceramic comprises firing a mix consisting essentially of:

Li,O-0.7 1.5 weight percent Na,0--8.3 8.9 weight percent MgO-0.5 2.0 weight percent Al O balance Preferably the ratio of Li O to MgO is in the range The invention will now be further described by way of example.

An intimate mix was prepared from the following ferent temperatures set out in the table below, the fir- TABLE 1 Firing Weight Temperature C percent NaAl OR From this table it can be clearly seen that the type NaAl O Beta-alumina content can be maintained at between and percent over a wide range of firing temperatures.

The resistivity and density of the sample discs prepared as above and fired at l,580 C were measured and the following results obtained:

density3.04 gms/cm resistivity--l.60 ohm cms at 350 C The fracture strength of these latter samples was measured at time intervals after storage in an atmosphere of 100 percent humidity. The following results were obtained.

Initial fracture strength 13,000 p.s.i.

After 1 week 13,000 p.s.i.

For comparison with the composition of the above example, sample Beta-alumina discs were prepared as above having the same MgO content but the Li O contents of 0.5 weight percent and 1.8 weight percent which are outside the range of the present invention. Using the same X-ray technique the relative proportions of NaAl O to NaAl O types of Beta-alumina were as given in Table 2 below.

TABLE 2 V Percent Firing Percent LizO Temperature NaAl Ou e ee 1550 62 1600 62 The fracture strength of the Beta-alumina having the 0.5% Li,0 content was measured in an atmosphere of 100% humidity and the following results obtained.

Initial fracture strength- 16,00 p.s.i.

After 24 hours-1,000 p.s.i.

It can be seen from these fracture tests that an Li,0 content within the range in accordance with the invention produces a stable Beta-alumina, whereas those outside the range are unstable.

We claim:

1. A method of preparing a Beta-alumina polycrystalline ceramic wherein the type NaAl O Beta-alumina content is between 90 and 100 percent comprising firing at a temperature from about 1,500C to about 1,630 C a mix consisting essentially of by weight: l i,0 0.7 1.5 percent Na 8.3-8.9 percent MgO 0.5- 2.0 percent M 0, balance.

2. A method as claimed in claim 1, wherein the ratio of M 0 to MgO is in the range 1:1 to 121.5.

3. A method as claimed in claim 1, wherein the U 0 content is 0.9 percent by weight.

4. A method as claimed in claim 1, wherein the MgO content is 1.0% by weight.

5. A Beta-alumina polycrystalline ceramic wherein the type NaAl O Beta-alumina content is between 90 and percent prepared by a method comprising firing at a temperature from about l,500 C to about 1,630 C a mix consisting essentially of by weight:

Li O 0.7 1.5 percent Na, 8.3 8.9 percent MgO 0.5 2.0 percent A1 0 balance.

6. The method 'of claim 1 wherein a precursor of Li O is employed.

7. The method of claim 6 wherein said precursor is LlNOs.

8. The method of claim 1 wherein a precursor of MgO is employed.

7 g 'llhe method of claim 8 wherein said precursor is MgCO MW 10. The ceramic of claim 5 wherein the ratio of Li2O to MgO isfrom 1:1 to 111.5. 

2. A method as claimed in claim 1, wherein the ratio of Li2O to MgO is in the range 1:1 to 1:1.5.
 3. A method as claimed in claim 1, wherein the Li2O content is 0.9 percent by weight.
 4. A method as claimed in claim 1, wherein the MgO content is 1.0% by weight.
 5. A Beta-alumina polycrystalline ceramic wherein the type NaAl5O8 Beta-alumina content is between 90 and 100 percent prepared by a method comprising firing at a temperature from about 1,500* C to about 1,630* C a mix consisting essentially of by weight: Li2O - 0.7 - 1.5 percent Na2O - 8.3 -8.9 percent MgO - 0.5 - 2.0 percent Al2O3 - balance.
 6. The method of claim 1 wherein a precursor of Li20 is employed.
 7. The method of claim 6 wherein said precursor is LiN03.
 8. The method of claim 1 wherein a precursor of Mg0 is employed.
 9. The method of claim 8 wherein said precursor is MgC03.
 10. The ceramic of claim 5 wherein the ratio of Li20 to Mg0 is from 1:1 to 1:1.5. 